The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor, and particularly, it relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, and a network camera.
In these years, there have been available some vehicles equipped with a plurality of cameras for a purpose of enhancing convenience and security. For example, in case of a vehicle equipped with a backup camera to take an image behind the vehicle, a driver can see the rear view of the vehicle on a monitor upon backing up the vehicle. Accordingly, the driver can safely move the vehicle backward without hitting an object even if such an object is in a shadow of the vehicle and not visible from the driver. Such a camera mounted on a vehicle, i.e., a so-called onboard camera, is expected to be continuously on demand.
Normally, an onboard camera is often accommodated in a relatively small space such as in a backdoor of the vehicle, a front grill, a side mirror, and inside of the vehicle. For this reason, an imaging lens to be mounted in such an onboard camera is required to have compatibility to high resolution resulting from a high-pixel density imaging element and to have a wide angle to be compatible with a wide imaging range, as well as a small size. However, it is difficult to attain a small size and compatibility with high resolution while satisfactorily correcting aberrations, and further attain a wide imaging angle. For example, when a size of an imaging lens is reduced, refractive power of each lens tends to become stronger, so that it is difficult to satisfactorily correct aberrations thereof. Therefore, upon actually designing the imaging lens, it is important to satisfy those demands in a balanced manner.
As an imaging lens that has a wide imaging angle, for example, there is known an imaging lens described in Patent Reference. The imaging lens includes a first lens that has a shape of a meniscus lens directing a convex surface thereof to an object side and is negative; a second lens having a biconcave shape; a third lens having biconvex shape; an aperture stop; a fourth lens having a biconvex shape; and a fifth lens that has a shape of a meniscus lens directing a concave surface thereof to the object side and is negative, arranged in the order from the object side.
According to the imaging lens disclosed in Patent Reference, the third lens and the fourth lens in the configuration are formed of high-dispersion materials. Accordingly, it is possible to correct the field curvature and the chromatic aberration of magnification. Further, the second lens is formed in a shape so as to be a biconcave lens near the optical axis thereof, so that the second lens has strong negative refractive power. Accordingly, it is possible to attain a wide angle.
Patent Reference Japanese Patent Application Publication No. 2011-107593
According to the imaging lens described in Patent Reference, it is achievable to have a wide angle in spite of a small number of lenses that is as small as five, and also to relatively satisfactorily correct aberrations. However, since a total length of the whole lens system tends to be long relative to a focal length. Accordingly, it is difficult to satisfy the recent demands for downsizing and there is a limit by itself to balance between the downsizing of the imaging lens and satisfactory aberration correction. Here, such an issue is not specific to the imaging lens to be mounted in the onboard camera, but it is rather a common problem for the imaging lens to be mounted in a relatively small camera such as cellular phones, digital still cameras, portable information terminals, security cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain a wide angle in spite of the small size thereof and can satisfactorily correct aberrations.
Further objects and advantages of the present invention will be apparent from the following description of the present invention.